1. Field of the Invention
This invention relates to slow motion video signal generation.
A slow motion video (television) signal is generated by increasing the number of fields of the original signal and displaying the resultant signal at the same field frequency as the original signal whereby an illusion of slowed motion is obtained. Conventionally, the increase in the number of fields is obtained by repeating fields of the original signal. If a repeated field is to be of the same interlace polarity, no alteration of the field is effected. If a repeated field is to be of opposite interlace polarity, it may be subjected to spatial interpolation (i.e. spatial interpolation between the lines thereof), to change its interlace polarity. Such spatial interpolation may be effected in a fixed direction. For example, the spatial interpolation may be effected in a purely vertical direction, i.e. it may comprise vertical line averaging performed, for example, by a 1/2:1/2 spatial (vertical) filter. This technique of fixed direction (e.g. vertical) spatial interpolation is subject to the following disadvantages.
1. There is a loss of apparent vertical resolution in that, due to the interlaced field system, each field contains only half of the vertical information of the picture. The loss of apparent vertical resolution gets worse as to speed is reduced until, at the limit, when the picture is still, half of the vertical resolution is lost.
2. There is an increase in apparent vertical aliasing due to the absence of a vertical prefilter prior to repetition of interlaced fields to prevent all spectral components above the Nyquist limit from being sampled.
3. The fact that some fields are subjected to spatial interpolation to change the interlace polarity, and some are not, gives rise to a beating effect which increases flicker of the displayed picture. This effect is particularly objectionable at slow motion speeds which are close to the original (normal) speed.
An alternative technique to field repetition which could, in principle, be used, would be to generate a new series of fields by linear temporal interpolation between the fields of the original signal. This, however, would require the use of prefilters in the vertical and temporal directions to prevent all spectral components above the Nyquist limit from being sampled, and therefore to prevent aliasing. This limitation on the spatial/temporal resolution would cause severe subjective impairments on picture motion, which would be seen as picture blur. Thus, this technique cannot by itself provide satisfactory results.
2. Description of the Prior Art
International (PCT) Patent Applications Publication Nos. WO85/02080 and WO85/04542 disclose the use of a movement detector in a television signal processing apparatus. WO85/02080 is concerned with "upconverting" an interlaced signal to a non-interlaced format in a satellite broadcast receiver, which is done by interpolating extra lines which are combined with the lines of the input field to provide a non-interlaced output. A movement detector monitors delayed lines and fields of the input signal and produces a signal which is applied to a field store processor which in turn produces a control signal for an interpolator (for interpolating the extra lines). WO85/02080 mentions that the movement detector can be based on an absolute frame difference signal; and mentions also that, in upconverting an interlaced signal, temporal interpolation is better when there is no movement, because it does not detract from vertical resolution, and that vertical interpolation is better in the case of movement, but causes loss of vertical resolution when there is no movement. WO85/04542 is concerned with the same type of system, and is concerned more specifically with the structure of the movement detector, which generates an absolute frame difference signal. Neither of the above documents is concerned with the provision of a slow motion system, in which, due to the fact that the display is in slow motion, generally for the purpose of close examination of viewers (e.g. to see who has won a running race), any degradation in picture quality will be more readily seen than in the case of a normal speed picture.